A nickel-hydride storage battery (hereinafter, also simply referred to as a battery) is generally configured such that a capacity of a negative electrode is set larger than a capacity of a positive electrode, so that a battery capacity is limited by the positive capacity (hereinafter, also referred to as positive electrode limitation). With this positive electrode limitation, it is possible to restrain an increase in internal pressure of a battery during overcharge and during overdischarge. It is to be noted that, by comparison between the negative electrode and the positive electrode, an excessive uncharged portion available for charge is referred to as “charge reserve” and an excessive charge portion available for discharge is referred to as “discharge reserve”.
Meanwhile, recent searches have revealed that some of nickel-hydride storage batteries allow a slight amount of hydrogen gas to continuously permeate through a battery case and leak to the outside of a battery. When the hydrogen gas leaks out of the battery, accordingly, hydrogen is released from hydrogen absorbing alloy of a negative electrode according to a hydrogen leakage amount in order to keep balance of hydrogen partial pressure in the battery case. Accordingly, a discharge reserve capacity of the negative electrode decreases. This hydrogen leakage advances very slowly and thus will not cause any trouble for a relatively short use period.
However, long-term use will cause deterioration in capacity balance between the positive electrode and the negative electrode and decrease the capacity of the negative electrode, causing a discharge reserve capacity of the negative electrode to run out. As a result, the nickel-hydride storage battery is subjected to negative electrode limitation (representing that a discharge capacity of a battery is limited by a negative electrode capacity), thus decreasing the battery capacity and resulting in large lowering of battery characteristics. In a case where a nickel-hydride storage battery is used as a power source for an electric vehicle, a hybrid vehicle, or the like, a long-term life is demanded and therefore this lowering of battery characteristics would be problematic.
To solve the above problems, there has been proposed a method for reproducing a nickel-hydride storage battery with a lowered battery capacity due to a decrease in the discharge capacity of the negative electrode by increasing (restoring) the discharge capacity of the negative electrode (see Patent Document 1). When the nickel-hydride storage battery is overcharged, electrons are released from the positive electrode and also oxygen gas is generated from hydroxide ions in the aqueous electrolyte. In the negative electrode, on the other hand, hydrogen generated by water decomposition is absorbed by the hydrogen absorbing alloy. However, oxygen gas generated from the positive electrode is normally consumed by reaction with the hydrogen absorbed by the hydrogen absorbing alloy (water is generated) (this action is also referred to as recombination reaction). Ultimately, it is impossible to increase the amount of hydrogen to be absorbed by the hydrogen absorbing alloy of the negative electrode by simply overcharging the battery.
In Patent Document 1, in contrast, the safety valve device is valve-opened in advance, and at least part of the oxygen gas generated from the positive electrode due to overcharge of the nickel-hydride storage battery is released out of the battery through the valve-opened safety valve device. Accordingly, in the battery, the hydrogen absorbed by the hydrogen absorbing alloy of the negative electrode in association with overcharge excessively increases relative to the oxygen gas. As a result, at least part of the hydrogen absorbed by the hydrogen absorbing alloy of the negative electrode by overcharge can be left as being absorbed in the hydrogen absorbing alloy without reacting with the generated oxygen gas (i.e., the discharge capacity of the negative electrode can be increased). This can restore the discharge capacity of the negative electrode.